Mesocyclone
A area of rotation of storm size that may often be found on the southwest part of a supercell. Its circulation can be larger than the tornado that may develop within it, but not necessarily. Originally a radar term for a rotation signature that met certain criteria, it is best seen on Doppler radar.
Storm
An individual low pressure disturbance, complete with winds, clouds, and precipitation. The name is associated with destructive or unpleasant weather. Storm-scale refers to disturbances the size of individual thunderstorms.
Supercell
A severe thunderstorm characterized by a rotating, long-lived, intense updraft. Although not very common, they produce a relatively large amount of severe weather, in particular, extremely large hail, damaging straight-line winds, and practically all violent tornadoes.
Circulation
The flow or motion of a fluid in or through a given area or volume. In meteorology, it is used to describe the flow of air as it moves around a pressure system in the atmosphere. It describes smaller patterns in semi-permanent pressure systems as well as the relatively permanent global currents of air. In oceanic terms, it is used to describe a water in current flow within a large area, usually a closed circular pattern such as in the North Atlantic.
Tornado
A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena. They can occur anywhere in the world given the right conditions, but are most frequent in the United States in an area bounded by the Rockies on the west and the Appalachians in the east.
Doppler Radar
Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar. Using the Doppler effect, it measures the velocity of particles. Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away.